1. Field of the Invention
The present invention relates generally to a magnetic recording medium, and more particularly to a magnetic recording medium, for example, a magnetic tape having a magnetic layer formed on a flexible non-magnetic base.
2. Description of the Prior Art
A conventional magnetic tape is manufactured in a way that a magnetic paint or composition containing predominantly magnetic powders and resinous binder is coated on a flexible non-magnetic film and an axis of easy magnetization of the magnetic powder is then oriented in a predetermined direction. The non-magnetic film to be used there is composed of polyolefines, for example, polypropylene; cellulose derivatives, for example, cellulose triacetate and cellulose diacetate; or vinyl resins, for example, polyvinyl chloride. However, materials widely used for the base film are polyesters, for example, polyethylene terephthalate and polyethylene naphthalate which are superior in dimensional stability, cutting strength and so on. The polyester film is drawn or elongated more intensely in the longitudinal direction than in the width direction. There is used as the magnetic powder gamma hematite (.gamma.-Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4), gamma hematite-magnetite (.gamma.-Fe.sub.2 O.sub.3 -Fe.sub.3 O.sub.4), magnetizable material having high coercive force obtained by addition of cobalt to the above iron oxides, ferromagnetic chromium dioxide (CrO.sub.2), acicular alloy powder, or these mixtures. The resinous binder is composed of thermoplastic resin, thermosetting resin, reactive resin, or these mixture.
Magnetic property or static characteristics of the magnetic tape prepared as described above depends mainly on the kind and performance of the magnetic powder. Physical and mechanical properties, however, of the magnetic tape mostly depend on the kind and perpormance of the non-magnetic film and performance of the magnetic layer itself. Recently, a magnetic tape has been widely used particularly for a video tape recorder (VTR) of helical scanning type because of the increased utility value. The VTR has come into more wide use since a so-called video cassette as a suitably-shaped casing receiving the magnetic tape was devised and used. In the VTR, the magnetic tape supported in the cassette is generally set in a loading state when used, in such a manner that it is drawn out of the cassette and then contacted with the circumference of a cylinder (head drum assembly) in a slant direction relative to a rotation plane of a rotating magnetic head. Under this condition, a recording or reproducing operation for the magnetic tape is effected with the magnetic head along the tracks slightly slanting relative to the longitudinal direction of the magnetic tape. Accordingly, the magnetic tape used in the video cassette is exposed to the loading state and severe use conditions, for example, a repeated running. For this reason, the magnetic tape must has a good mechanical strength and maintain a stable running property when used for a long time.
In such video cassette, it is desired that the amount or volume of the magnetic tape received therein is increased to effect a recording operation for a long time. However, it is in practice impossible to enlarge the size of the cassette so much. Accordingly, it is necessary to thin the magnetic tape itself so as to increase the amount to be received in the cassette. For example, the non-magnetic film of the magnetic tape is required to be as thin as possible. However, as a result of various experiments by the inventors, it has been found that the thinning of the magnetic tape by the use of the conventional technique results in a deterioration of a contacting condition (hereafter called "a head contact") between the magnetic tape and the magnetic head so that the magnetic tape can not be used in practice. The experimental results on the head contact regarding the conventional thinned magnetic tape are shown in the following Table I. In this case, the thickness of each magnetic layer is 4 microns and the base film is composed of polyethylene terephthalate. And when a Young's modulus of the base film and the magnetic layer in the longitudinal direction is represented by "LDE" and a Young's modulus of them in the width direction is represented by "TDE", a ratio of Young's moduluses (LDE/TDE) is 1.8 for the base film and 1.9 for the magnetic layer.
TABLE I ______________________________________ Thickness of Base Film Head Contact ______________________________________ 10.mu. C 14.mu. B 16.mu. A ______________________________________
In this Table, "A" shows a good result that a strong and flat output signal is obtained when an output signal or wave form on the reproducing operation is observed per one picture, "B" shows a not so good result that the output signal is distorted upward and downward at the central portion, and "C" shows a bad result that the output signal is weakened and distorted. These letters A, B and C defined as above are used in the same meaning also in the below description.
As apparent from the Table I, the head contact is deteriorated as the base film is thinner, which causes an unfavourable condition. The reason of the deterioration of the head contact is that the rigidity of the base film in the width direction is relatively lesser or the distortion of the base film in the same direction is larger due to the larger elongation degree or Young's modulus in the longitudinal direction than in the width direction. This causes a large distortion or deformation of the base film in the width direction when the magnetic head slides in the transverse direction to the longitudinal direction of the base film, so that the magnetic tape is distorted as if it were pushed up on the magnetic head. As a result, a spacing loss between the magnetic head and the magnetic layer occurs momentarily to deteriorate the head contact. Under this condition, particularly, a stable running property of the magnetic tape is obstructed, the magnetic tape can not endure a long-time use and further the electro-magnetic conversion characteristic is deteriorated.